It has long been common practice to provide an internal support for pipe while the pipe is being bent to avoid wrinkling and other non-uniformities in the pipe normally resulting from the bending of pipe without adequate internal support during bending. One of the problems encountered in bending mandrels is the adjustments which have to be made as a result of slight variations in the diameter of the pipe. Most prior art mandrels have used actuators with a past-center type of expander so that the outside diameter of the expanded mandrel is a single dimension and even though some accommodation is provided by having resilient pads engaging the pipe interior, all but the most minor changes in diameter require some shimming.
U.S. Pat. No. 2,401,052 dated May 28, 1964 to J. D. Cummings, No. 3,043,361 dated July 10, 1962 to C. L. Kelso and No. 3,109,477 dated Nov. 5, 1963 to T. W. Avera are examples of pipe bending mandrels of the prior art.